1. Field of the invention
The present invention relates generally to an electrophotographic copying machine and, more particularly, to one which has made it practicable to produce positive prints from both positive and negative originals.
2. Prior art
As a variety of electrophotographic copying machines in the copying method wherein a static latent image formed on the surface of a static latent image carrier is formed into a toner image by supplying an electrified toner thereto from a developing device and a sheet fed to a position between said static latent image carrier and a transfer device placed facing the image carrier is charged with a polarity differing from that of said toner by said transfer device so as for said toner image to be transferred onto said sheet, there have conventionally been available copying machines which are capable of producing positive images from positive originals (hereinafter referred to as "P - P copying") as well as from negative originals (hereinafter referred to as "N - P copying"). For this purpose the copying function of these copying machines is based on either of the following methods.
One method, as disclosed in Japanese Patent Publication No. 176066/1985, includes the use of two developing units containing toners. Each toner contained in each developing unit has a tendency to be charged with a polarity different from that of the other when coming into contact with a carrier. The method also includes the use of two electrical transformer units for transfer charger for charging said sheet with a polarity different from that of the toner.
The other method, as disclosed in U.S. Pat. No. 3,877,803, includes the use of a static latent image carrier sensitive to both positive and negative polarities and the use of a transformer for an electrifying charger and a polarity alternating switch, the latter two devices for changing the voltage polarities applied to the electrifying charger according to whether the copying is from a positive or a negative original.
The former method, however, requires that the copying machine has at all time two developer units and two transformer units for transfer chargers as integral parts, and therefore, a larger space is required for these devices in the copying machine as opposed to the generalized pursuit of compactness and reduction in size. Moreover, the application of the two methods requires expensive high-voltage relays for switching the connection of the transfer charger with one of two electrical transformer units or for alternating the voltage polarities of the electrical transformer, and switches for controlling them. This causes the problem of making the mechanism complicated and more costly.
In the latter method, it was a problem that the static latent image carriers available for the purpose are limited in type, requiring selection from a small variety, and are high in cost as well.
Generally, the copying method practiced by general users is P - P copying, while N - P copying is practiced rarely and in a limited number of application fields.